The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known internal combustion engines operating at lean air-fuel ratios can reduce fuel consumption with decreased NOx emissions. Known exhaust aftertreatment systems for internal combustion engines operating lean of stoichiometry can include an oxidation catalytic converter, a lean-NOx reduction catalyst, also referred to as a NOx adsorber, a selective catalytic reduction (SCR) catalytic device, and a particulate filter. Known SCR devices promote catalytic reaction of NOx with a reductant, e.g., hydrocarbon, to produce nitrogen and water. Reductant is dispensed into an exhaust gas feedstream upstream of the SCR device.
A known scheme to reduce NOx emissions includes injecting hydrocarbons upstream of a hydrocarbon-selective catalytic reduction (HC-SCR) device with an exhaust gas feedstream that is oxygen-rich. One HC-SCR device includes using a catalyst including alumina-supported silver, e.g., Ag/Al2O3, to selectively reduce NOx under lean exhaust conditions using hydrocarbon as a reductant. Known hydrocarbon reductants include short-chain hydrocarbons (e.g., propene, propane) and long-chain hydrocarbons (e.g., octane, decane). NOx reduction strategies using short-chain hydrocarbons present in engine exhaust as a reductant for reducing NOx emissions at higher temperatures are known. NOx reduction strategies using long-chain hydrocarbons and oxygenated hydrocarbons (e.g., ethanol) present in engine exhaust as reductants to reduce NOx emissions at lower temperatures are known.
Systems using NOx adsorbers can require catalytic devices having large displaced volumes and large masses of platinum-group metals coupled with use of low sulfur fuel to operate efficiently. Known NOx adsorbers require periodic catalyst regeneration that can include injecting fuel into the exhaust gas feedstream to generate high exhaust gas temperatures and dispensing reductants to regenerate the storage material of the catalyst.